1. Field of the Invention
The present invention relates to power transfer systems for controlling drive torque distribution, and more particularly to a torque vectoring gear drive apparatus including a planetary differential unit and a speed gear assembly for selectively varying a gear transmission ratio between an input shaft and output shafts.
2. Description of the Prior Art
Four-wheel drive (4WD) motor vehicles are becoming increasingly popular, especially “full-time” four-wheel drive systems called in the art as all-wheel drive (AWD) systems. In such AWD systems, torque transfer cases are typically provided with an inter-axle differential for dividing torque between front wheels and rear wheels of the motor vehicle. The inter-axle differential enables the front wheels and the rear wheels to rotate at different speeds, which occurs during normal turning of the motor vehicle or in the event that the front wheels and the rear wheels have tires with different diameters. However, to prevent excessive slipping between the front and the rear wheels, as might occur when one set of wheels encounters a low-traction condition, such as ice, these transfer cases typically includes a selectively engageable clutch which is operative to lock the inter-axle differential upon sensing a predetermined amount of slippage between the front output shaft and the rear output shaft of the transfer case. Locking of the inter-axle differential prevents any relative slip or differentiation between the front output shaft and the rear output shaft of the transfer case.
Known prior AWD systems have generally required complex electronic sensors or other complex systems to monitor the slip or differentiation between the front output shaft and the rear output shaft or the front wheels and the rear wheels of a motor vehicle. Upon sensing slip or differentiation, an electronic control system determines whether the slip or differentiation being encountered is within a “normal” expected range or is “excessive.” If the electronic control system indicates that the slip or differentiation being experienced is “excessive,” the electronic control system causes the selectively engageable clutch to lock the inter-axle differential to preclude any further slip or differentiation.
Furthermore, currently many AWD vehicles are equipped with various vehicle stability systems, which helps correct vehicle understeer and/or oversteer to help keep the vehicle on course and avoid a collision. Typically, the known vehicle stability systems monitor the vehicle behavior based on a number of electronic sensors, and decrease engine power and/or selectively apply brakes to the appropriate wheels as needed for stabilizing the vehicle trajectory. More specifically, if road conditions cause your vehicle to understeer, the vehicle stability system decreases engine power to correct the problem. If the understeer is not reduced, the vehicle stability system applies brakes to the inside front wheel to help turn the car back onto its intended course. This reduction in understeer helps the vehicle regain traction to get it back on course. Alternatively, if the vehicle begins to oversteer, the vehicle stability system automatically helps correct the problem by applying brakes to the outside front wheel. This helps to reverse the potential spin and puts the vehicle back on track.
The electronic control system and the vehicle stability system of this type can be expensive to manufacture and maintain and a more cost-effective, simplified system of limiting slip differentiation between the front output shaft and the rear output shaft and the vehicle stability system of a four-wheel drive torque transfer case in a motor vehicle would be desirable. Moreover, while known transfer cases, including but not limited to those discussed above, have proven to be acceptable for various vehicular driveline applications, such devices are nevertheless susceptible to improvements that may enhance their performance and cost. With this in mind, a need exists to develop improved hydraulic transfer case of AWD vehicles that advance the art.